1. Field of the Invention
This invention relates generally to greensheet articles and more particularly, in a preferred embodiment, to a flexible ceramic/organic web and methods for forming such a web that is capable of being fused at elevated temperatures and converted to ceramic articles of a variety of configurations.
2. Description of the Prior Art
In prior art flexible ceramic/organic web articles, it is well known to use open-celled organic sponges or foams as carriers of refractory ceramic particles impregnated thereon. Various admixtures of ceramic particles and impregnating techniques have been employed to produce ceramic/organic webs which are particularly useful for making fused ceramic structures or sintered parts some of which having relatively complex shapes which retain their structural integrity over wide temperature ranges. The major difficulty encountered by the use of these prior art methods have been loading sufficient quantities of ceramic slurry in a structure suitable for providing a product which is a strongly bonded material after sintering.
One method for forming ceramic articles is described in U.S. Pat. No. 4,075,303 issued Feb. 21, 1978 to Yarwood et al. There, a process for producing a ceramic article comprises providing an open celled organic polymer foam material web possessing a predetermined permeability and resilience, and impregnating the web with a fluid aqueous slurry of a thixotropic ceramic composition; then passing the impregnated foam material through a pair of rollers of a particular nip, the pair of rollers and the nip being preset to effect temporary successive compressions, amounting to about 50 to 90% of the thickness of said material for a first pass and 70 to 90% of the thickness for the second pass. Following the combined impregnation and compression steps, the resulting uniformly impregnated foam material is heated to remove moisture and then to volatilize the foam web component. The resulting article is then ready for use preferably as a filter for molten metals or optionally to be formed into ceramic articles by further heating to sinter the ceramic material.
This method of forming articles, however, is primarily for forming a permeable structure suitable for filtering molten metals. This technique appears to be an inefficient method for providing sinter ceramic articles because excessive amounts of work would be required to effectively load the open-celled foam structure to the degree of compactness required to permit forming sintered articles that exhibit low surface flaws and high structural uniformity over wide ranges of temperatures.
Another method for loading a foam web with ceramic material is described in U.S. Pat. No. 3,845,181 dated Oct. 29, 1974 issued to Ravault et al. There, the problems associated with loading foam with a slurry of finely divided ceramic particles are addressed. It is recognized that the strength of porous ceramic materials so manufactured is often not very great. This is believed to stem from the fact that during firing, the original organic foam structure burns out to leave thin, very fragile, webs of ceramic material webs. In the case of a reticular organic foam, ceramic tubes will be formed; in the case of a foam with cell walls, fragile ceramic part spheres which are very weakly bonded will occur. In either case, the structure is not adapted to withstand loading. Hence to solve the loading problem using foam and the slurry of finely divided ceramic particles, certain additives are added to the foam and/or the slurry prior to or during impregnation which are capable of attacking the foam and disintegrating it. After impregnation with the ceramic slurry and during drying of the materials and when the temperature is increased, the concentration of the attacking agent rises as a consequence of the evaporation of the slurry; the foam is attacked rapidly, and the dried (but unfired) structure tends to become a coherent foam-structured mix of finely divided particulate ceramic material and organic foam breakdown by-products, rather than a foam coated with a layer of particulate ceramic. Hence, on firing, a solid but more coherent overall structure is formed.
It should be recognized that foams and sponges present inherent disadvantages as a medium for high level loading of organic filler materials. Also, the procedural steps required to load filler materials in open-cell structure foam are not easily adaptable to continuous or semi-continuous mass production techniques because of the ease by which foam and sponge materials may be compressed and deformed.
A further prior art method for storing refractory materials on a carrier which employs, in addition to foam, a mesh or a cloth as the carrier of the refractory pigment is described in U.S. Pat. No. 3,111,396 dated Nov. 19, 1963 to B. B. Ball. There, a slurry is made of a liquid such as an organic solvent or water and a finely divided powder such as metal, metal oxide, or other metal compounds. The slurry can include an organic binder such as a synthetic resin. The selected organic structure is then saturated with the slurry as thoroughly as possible. Any excess slurry is removed from the pores or open portions of the structure by squeezing, leaving only the matrix of the organic structure impregnated with the slurry. Then the coated web is slowly dried at or near room temperature e.g., for about 18 hours to assure slow liquid removal in order to keep the powder of the slurry in place. Such a web coating procedure appears to be a very slow process and is not very well suited for mass production techniques for making greensheets.
Another method for forming a particle sheet material is described in U.S. Pat. No. 3,962,389 dated June 8, 1976 to Takase et al. Takase et al teaches forming a ceramic greensheet material which does not contain organic binders such as elastomers and latex, fixatives and polymeric coagulating agents. Their method is one for producing particle sheet material which is shapable and contains large amounts of inorganic fine particles, wherein the method steps comprises graft copolymerizing cationic monomer to cellulose pulp to obtain cationic pulp causing the cationic pulp to adsorb and coagulate the inorganic particles which has a negative interfacial electrokinetic potential in water, and then forming the particle sheet material therefrom by using a sheet forming machine. Although this method of Takase et al seems to be a very highly efficient method for loading particles in a sheet structure it does not appear to be an effective method for mass producing greensheets since the abrasive nature of particles would probably destroy vital components of the sheet forming equipment after a short period of time.
The present invention is directed to the improved techniques and methods for forming a ceramic/inorganic web the product which resulted from confronting and solving the basic material storing problems as described above. In the course of the development additional unforeseen problems were resolved as will be apparent.